Diesel engines typically include exhaust gas after-treatment systems that receive and treat exhaust gas from the engine. Exhaust gas after-treatment systems can include many elements, but most commonly include at least one diesel oxidation catalyst (DOC), particulate matter filter, and selective catalytic reduction (SCR) system. The SCR system reduces nitrogen oxides (NOx) present in the exhaust gas stream prior to the stream exiting the after-treatment system. In order to reduce NOx, a reductant such as a urea solution or an ammonium formate solution is added to the exhaust gas before the exhaust gas reaches the SCR system. The reductant decomposes to produce a substance, such as ammonia, prior to passing through a catalyst of the SCR system. The ammonia, or other substance, reacts with NOx in the exhaust stream, in the presence of the SCR catalyst, to reduce NOx to less harmful emissions, such as N2 and H2O.
For proper operation, however, the reductant must decompose properly prior to entering the SCR catalyst. For proper decomposition of the reductant in an exhaust gas stream, the temperature of the exhaust gas stream must be at or above certain high temperature thresholds until decomposition is complete. During normal operating conditions, the temperature of the exhaust gas in isolated pockets can be sufficiently high to properly decompose the reductant. However, isolated cold spots or pockets in the exhaust gas where the exhaust gas temperatures are not sufficiently high to decompose the reductant are often present in the exhaust gas. If reductant is injected into such cold spots, un-decomposed reductant buildup and solidification on the reductant injector and/or walls of the after-treatment system may occur. Heating the entire exhaust system to prevent cold spots is not a cost-effective approach and in certain applications, may not be feasible due to design constraints.
A more efficient and effective apparatus, system, and method are needed to reduce cold spots in an exhaust gas stream into which a reductant is injected.